Neuromyelitis optica (NMO) is a rare, disabling and sometimes fatal, central nervous system (CNS) autoimmune inflammatory demyelinating disease. Aquaporin-4 (AQP4), a water channel that is expressed abundantly on astrocytes, is the primary target in NMO. A majority of NMOSD patients have AQP4-specific Abs, which are IgG1, a T cell-dependent subclass, indicating that AQP4-specific T cells have a key role in this humoral autoimmune disease. Certain clinical and pathologic findings suggest that Th17 cells participate in NMO pathogenesis. Our group first identified AQP4-specific T cells, an observation that was confirmed by other groups. We observed that T cells specific for dominant AQP4 epitopes exhibited Th17 polarization, providing further support that Th17 cells participate in NMO pathogenesis. Further, the dominant AQP4 T cell epitope identified in NMO contains a ten amino acid (aa) sequence that shares 90% homology to an aa sequence within an ABC-TP in Clostridium perfringens, a ubiquitous anaerobic gram-positive bacterium found in human commensal gut flora. This discovery, along with the demonstration that certain Clostridia species in the gut of humans can regulate the balance between regulatory T cells (Treg) and Th17 cells, suggest that gut microbiota, and possibly C. perfringens itself, could participate in NMO pathogenesis. Recently, we evaluated the gut microbiome in NMO, multiple sclerosis (MS) and healthy controls (HC). Remarkably, C. perfringens was the second most significantly enriched taxon in NMO, and among bacteria identified at the species level, C. perfringens was the one most highly associated with NMO. Thus, we have hypothesized that C. perfringens may have dual functions in NMO pathogenesis: it may (1) serve as its own proinflammatory adjuvant, promoting Th17 polarization, and (2) expose a determinant of a Clostridium ABC-TP that cross-reacts with AQP4, leading to expansion of AQP4-reactive T cells. Here, we propose to colonize germ-free mice with gut microbiota from NMO patients or by mono-colonization with C. perfringens, in order to directly examine the potential role of gut microbiota in NMO and C. perfringens, respectively, in proinflammatory T cell differentiation. In the process of examining our hypothesis regarding C. perfringens, our study should provide important information regarding the potential role of gut microbiota in NMO and CNS autoimmunity in general.